Apache shows the first site in alphabetic order that exists for a specific IP address if no site matches. So if you want to create a default site, you just have to ensure that it is the first one in alphabetic order.

Create a new site in ispconfig and use as domain name e.g.:

00000-defaultsite.tld

The domain name does not have to exist, it just have to be the first one to become the default site.

Then you most likely did not select the correct IP address for the site or you did not wait until changes were written to disk. The steps I described abive work for all Linux distributions and setups that have a apache server installed.

The redirect is done for the domain of the website. As you dont use a real domain for the site, the redirect can not work. You can e.g. add a .htaccess file with a wildcard redirect in the web directory of the site.

I agree with all of the above but let me tell my thoughts here.
a) The Web site is intentionally disabled by a moderator -> The user goes into the default site as Apache default behavior is. - I agree
b) The web site is disabled due to another reason such as bandwidth limitation - The user goes into the default site as Apache default behavior is. - I do NOT agree
I beleive in such cases ispconfig could redirect the user into a bandwidth page with the specific error message. Such an action could be established by creating a folder in the main web like /web/blocking_pages and the specific host would then point into that page instead of /web/